1. Field of the Invention
The present invention relates to brush seals for sealing high pressure fluid areas from low pressure fluid areas, the sealed fluid being a gas or a liquid.
2. Description of the Related Art
Over the last decade, brush seals have emerged as a very promising technology for sealing high pressure areas from low pressure areas, such as those found in gas turbine engines.
Typically, brush seals inhibit flow of a gas (or liquid) in a stream along a shaft. The gas (or liquid) is sealed, for example, within a machine housing which has a system pressure. The outside area of the machine housing, toward which the gas (or liquid) will tend to leak, has a discharge pressure. The system pressure is greater than the discharge pressure, creating a lateral pressure differential. The system pressure side of the brush seal is referred to as the high pressure side, while the discharge pressure side of the brush seal is referred to as the low pressure side.
Conventional brush seals usually include an annular retaining plate, an annular back plate, and a plurality of flexible bristles secured between the annular retaining plate and the annular back plate. The plurality of flexible bristles, otherwise called a bristle pack, extend inwardly from bristle roots at an outer peripheral edge to free ends or tips. The tips of the plurality of flexible bristles run near to or contact the shaft to seal the system pressure from the discharge pressure.
As described in U.S. Pat. No. 5,401,036 (the '036 patent), conventional brush seals exhibit "hysteresis" and "pressure stiffening" effects, which tend to adversely affect the seal performance and life. These effects are considerably attenuated, if not eliminated, by incorporating a "recessed back plate" described in the '036 patent.
A difficulty with the "recessed back plate" brush seal, however, is that it has a tendency to wear unevenly. The flexible bristles on the system pressure side tend to wear to a greater extent than the bristles on the discharge pressure side of the seal for reasons described in detail below. The present invention substantially overcomes this difficulty in the prior art.